It has been proposed heretofore to form a concentrate or inspissate from a solution by freezing crystals of a solvent component therefrom by contacting the solution with a surface which is cooled, i.e. by passing a coolant, i.e. a refrigerant or a cold carrier such as brine, through the cooling surface which is in contact with the solution.
When this technique is applied to the concentrating of aqueous solutions, crystals of ice form upon the cooled surface and must be removed to prevent the ice layer from blocking further cold transfer to the solution. It is recognized in the art that ice constitutes a thermal insulator which becomes interposed between the solution and the cooled surface and can render the system inefficient.
It is known to remove the ice crystals from the surface or to prevent their deposition thereon by relative vibration of the surface and the solution, i.e. by vibrating the cooled surface to establish shear forces at the interface between the ice and/or liquid and the cooled surface. As a result, the ice does not coat the surface of the cooled body to any significant extent and the effect of the chilling or cooling can be maintained.
In the aforementioned copending application, there is described a method of and an apparatus for carrying out a thickening of a solution by an improved technique whereby the solution is introduced between surfaces which are cooled in the manner described and relative vibration is established, the solution passing between the surfaces in a direction transverse to the gravitational force so that the difference in density between the solution to be concentrated and the crystals which are formed by cooling causes the crystals to rise to the surface or to descend to the bottom of the vessel and hence provides for a continuous removal of the crystals from the solution.
The present invention is an extension of the principles described in that application.
It has been found in practice that, with still earlier systems for the aforedescribed purpose, uniform and continuous vibration of the cooling surface creates problems of various types. For example, the tendency of ice to freeze to the surface is not generally constant over the entire surface because different portions of the surface contact portions of the solution of different concentration. As a result, the temperature at which the solvent component can be frozen out of the solution tends to decrease as the concentration of the solution increases and hence parts of the cooled surface or some of the cooled surfaces must be maintained at lower temperatures than other parts, increasing the tendency of crystal deposits to form on these surfaces. A vibration that may be optimum for the removal of crystals from a solution of low concentration may not be optimum for the removal of crystals of the solvent in regions of high concentration.
The problem cannot be overcome by simply increasing the degree of vibration throughout the system since such excessive vibration also has been found to pose problems, especially in decreasing the efficiency of heat abstraction from the solution at the solid interface.
Thus it has been difficult, if not impossible, with earlier systems to effect a uniform freezing of crystals from a solution of the type described at an optimum rate.
Furthermore, when high oscillation frequencies of the heat-exchange surfaces are used, as has been deemed necessary heretofore when solutions of low concentrations are to be subjected to freeze out concentration, the conventional process creates the danger that, on the side of the heat-exchange surface in contact with the refrigerant or the cold carrier, cavitation phenomena may develop with a consequent reduction in heat abstraction efficiency and reduction in the freezing rate. Furthermore, high frequency oscillations may apply excessive stress to the vibrating components to cause problems with the apparatus.
Still another difficulty with prior art processes has been found to lie in the nonoptimum utilization of the energy operating the system and, more particularly, the energy supplied in the form of vibration to the system. At the beginning of the freezing process, as has been described, because of the low concentration of the solution, a high heat transfer rate is observed. Hence there is an increase in the tendency for the frozen-out or crystallized component to adhere to the heat exchange surface so that relatively high oscillation frequencies must be used to overcome this disadvantage. On the other hand, high oscillation or vibration frequencies may not be desirable elsewhere and hence the energy utilization or energy efficiency at other points in the apparatus may be less than desirable.